3D Displays, Continued
Frame Sequential (Alternate Frame) Display and Active Shutter Glasses
Some of the latest-generation 3D displays, televisions, and projectors are capable of displaying 3D video with separate left- and right-eye pictures in an alternating sequence. To avoid flicker, a refresh rate of 120 Hz or higher is used. A 120 Hz 3D monitor displays a full-resolution frame for one eye for a 120th of a second, followed by a full-resolution frame for the other eye for the next 120th of a second. Each eye will see 60 frames per second, but for less than half the time that the video is playing.
Note that a frame sequential display does not need to be modified with a polarizing filter. It only needs to be able to display frames at a rate high enough to avoid the appearance of flicker (generally, 60 Hz or higher for each eye is required to avoid noticeable flicker). As polarizing filters can affect the overall image quality, frame sequential displays, TVs, and projectors will typically have better image quality than polarized displays (for both 3D content and normal 2D content).
Frame sequential displays (also known as alternate frame displays) are often paired with liquid crystal (LC) active shutter glasses for viewing 3D content. Active shutter glasses, such as Nvidia’s 3D Vision glasses, use liquid crystal lenses in front of each eye. The glasses receive an infrared synchronization signal from a base station. This signal is used to synchronize the glasses to the display, such that the left eye is blocked when a frame of video is being displayed for the right eye, and vice versa. Note that there is a “blanking interval” during the transition from one frame to the next where the active shutter glasses block both eyes.
Active shutter glasses provide a number of benefits, including:
- Full resolution is possible. Because frames are displayed sequentially, each frame is able to use the full resolution of the display. Active shutter glasses are capable of providing a picture quality that has twice the pixel resolution of 3D displays that rely on polarized glasses.
- Extremely low crosstalk. As each frame is displayed, the lens in front of the corresponding eye is given an electrical signal that makes the lens transparent. When the glasses are tuned for and synchronized perfectly to the display, each eye will see very little of the image meant for the other eye. This results in a very sharp and clear 3D picture.
- LC shutter glasses are not sensitive to your head position and viewing angle, whereas polarized glasses suffer from image quality problems if you are not positioned directly along the center of the direction that the display is facing.